This invention relates generally to magnetic disk drives, more particularly to spin valve--magnetoresistive (MR) thin film read heads, and most particularly to methods and structures for providing a pinning mechanism for spin valve sensors.
Magnetic disk drives are used to store and retrieve data for digital electronic apparatus such as computers. In FIGS. 1A and 1B, a magnetic disk drive D of the prior art includes a sealed enclosure 1, a disk drive motor 2, a magnetic disk 3, supported for rotation by a spindle S1 of motor 2, an actuator 4 and an arm 5 attached to a spindle S2 of actuator 4. A suspension 6 is coupled at one end to the arm 5, and at its other end to a read/write head or transducer 7. The transducer 7 is typically an inductive write element with a sensor read element. As the motor 2 rotates the disk 3, as indicated by the arrow R, an air bearing is formed under the transducer 7 to lift it slightly off of the surface of the disk 3. Various magnetic "tracks" of information can be read from the magnetic disk 3 as the actuator 4 is caused to pivot in a short arc as indicate by the arrows P. The design and manufacture of magnetic disk drives is well known to those skilled in the art.
The most common type of sensor used in the transducers 7 is the magnetoresistive sensor. A magnetoresistive (MR) sensor is used to detect magnetic field signals by means of a changing resistance in a read element. A conventional MR sensor utilizes the anisotropic magnetoresistive (AMR) effect for such detection, where the read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. When there is relative motion between the MR sensor and a magnetic medium (such as a disk surface), a magnetic field from the medium causes a change in the direction of magnetization in the read element, thereby causing a corresponding change in resistance of the read element. The change in resistance can be detected to recover the recorded data on the magnetic medium.
Another form of magnetoresistance is known as spin valve magnetoresistance (SVMR). In a spin valve sensor, two ferromagnetic layers are separated by a copper layer. One of the ferromagnetic layers is a "free" layer and the other ferromagnetic layer is a "pinned" layer. In the prior art, this pinning has typically been achieved with an exchanged-coupled anti-ferromagnetic layer deposited adjacent to the pinned layer.
More particularly, and with reference to FIG. 1C, a shielded, single-element magnetoresistive head (MRH) 10 includes a first shield 12, a second shield 14, and a spin valve sensor 16 disposed within a gap (G) between shields 12 and 14. An air bearing surface S is defined by the MRH 10. The spin valve sensor is preferably centered in the gap G to avoid self-biasing effects. Lines of magnetic flux impinging upon the spin valve sensor create a detectable change in resistance. The design and manufacture of magnetoresistive heads, such as MRH 10, are well know to those skilled in the art.
In FIG. 2 a cross-sectional view taken along line 2--2 of FIG. 1 (i.e. from the direction of the air bearing surface S) illustrates the structure of the spin valve sensor 16 of the prior art. The spin valve sensor 16 includes a free layer 18, a copper layer 20, a pinned layer 22, and anti-ferromagnetic (AFM) layer 24. The spin valve sensor 16 is supported by a substrate 17 and a buffer layer 19. Ferromagnetic end regions 21 abut the ends of the spin valve sensor 16. Leads 25, typically made from gold or another low resistance material, bring the current to the spin valve sensor 16. A capping layer 27 is provided over the AFM layer 24. A current source 29 provides a current I.sub.b to flow through the various layers of the sensor 16, and signal detection circuitry 31 detects changes in resistance of the sensor 16 as it encounters magnetic fields.
The free and pinned layers are typically made from a soft ferromagnetic material such as Permalloy. As is well known to those skilled in the art, Permalloy is a magnetic material nominally including 81% nickel (Ni) and 19% iron (Fe). The layer 20 is copper. The AFM layer 24 is used to set the magnetic direction of the pinned layer 22, as will be discussed in greater detail below.
The purpose of the pinned layer 22 will be discussed with reference to FIGS. 3A and 3B. In FIG. 3A, the free layer 18 can have a magnetization direction as illustrated by the arrow 26, while the pinned layer 22 is magnetized as indicated by the arrow 28. Absent the magnetostatic coupling of the pinned layer 22, the ferromagnetic exchange coupling through the copper layer 20, and absent the field generated by the sensing current I.sub.S, the free layer 18 may have a magnetization as indicated by the dashed arrow 30. The actual magnetic angle 26 is the sum of the magnetic angle 30 and the magnetostatic coupling of the pinned layer 22, the ferromagnetic exchange coupling though the copper layer 20, and the field generated by the sensing current I.sub.S.
As seen in FIG. 3B on the curve R vrs. H, the magnetization 28 of the pinned layer 22 at a right angle to the magnetization 30 of the free layer 18 biases the free element to a point 32 on the curve that is relatively linear, and which has a relatively large slope. Linearity is, of course, desirable to provide a linear response, and the relatively large slope is desirable in that it produces large resistance changes in response to the changes in the magnetic field.
The anti-ferromagnetic material of the AFM layer 24 is either a manganese (Mn) alloy such as Iron-Manganese (FeMn) or an oxide such as Nickel-Oxide (NiO). The AFM layer 24 prevents the magnetization of the pinned layer 22 from rotating under most operating conditions, with the result that only the magnetic moment of the free layer 18 can rotate in the presence of an external magnetic field.
The SVMR sensor that has the most linear response and the widest dynamic range is one in which the magnetization of the pinned ferromagnetic layer 22 is parallel to the signal field and the magnetization of the free layer 18 is perpendicular to the signal field. However, the use of the AFM layer 24 to pin the pinned layer 22 presents several problems. For one, the exchange field strength generated by the AFM is highly sensitive to temperature. As the temperature increases, the AFM "softens" and its ability to fix the magnetization of the pinned ferromagnetic layer decreases. In consequence, SVMR sensors are highly sensitive to electrostatic discharge (ESD) currents and the resultant heating of the AFM 24. Further, AFM materials such as Fe-Mn are much more susceptible to corrosion than the other materials used in the SVMR sensor. The sensitivity of the AFM materials requires careful control of the fabrication process steps and the use of the protective materials for SVMR. AFM films 24 are also difficult to manufacture, in that they may require high annealing temperatures to be in the proper crystallographic antiferromagnetic phase.